Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cells in the eye. Organisms with tetrachromacy are called tetrachromats.

Tetrachromacy is demonstrated among several species of birds, fish, amphibians, reptiles, insects and some mammals.[2][3] It was the normal condition of most mammals in the past; a genetic change made the majority of species of this class eventually lose two of their four cones.[4][5]

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The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells, which are lower intensity light receptors) with different absorption spectra. This means that the animal may see wavelengths beyond those of a typical human being's eyesight, and may be able to distinguish between colors that, to a normal human, appear to be identical. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species.[6]

Some species of birds, such as the zebra finch and the Columbidae, use the ultraviolet wavelength 300–400 nm specific to tetrachromatic color vision as a tool during mate selection and foraging.[9] When selecting for mates, ultraviolet plumage and skin coloration show a high level of selection.[10] A typical bird eye will respond to wavelengths from about 300 to 700 nm. In terms of frequency, this corresponds to a band in the vicinity of 430–1000 THz. Most birds have retinas with four spectral types of cone cells that are believed to mediate tetrachromatic color vision. Bird color vision is further improved by the filtering of pigmented oil droplets that are located in the photoreceptors. The oil droplets filter incident light before it reaches the visual pigment in outer segments of the photoreceptors.

The four cone types, and the specialization of pigmented oil droplets, give birds better color vision than that of humans .[11][12] However, more recent research has suggested that tetrachromacy in birds only provide birds with a larger visual spectrum than that in humans (humans cannot see ultraviolet light, 300-400 nm), while the spectral resolution (the "sensitivity" to nuances) is similar.[13]

In areas where reindeer live, the sun remains very low in the sky for long periods. This means the light is scattered such that the majority of light that reaches objects is blue or UV. Some parts of the environment absorb UV light and therefore to UV-sensitive reindeer, appear to be black, strongly contrasting with the snow. These include urine (indicating predators or competitors), lichens (a food source) and fur (as possessed by wolves, predators of reindeer).[16] Although reindeer do not possess a specific UV opsin, retinal responses to 330 nm have been recorded, mediated by other opsins.[17] It has been proposed that UV flashes on power lines are responsible for reindeer avoiding these structures because "...in darkness these animals [reindeer] see power lines not as dim, passive structures but, rather, as lines of flickering light stretching across the terrain."[18]

In humans, two cone cell pigment genes are present on the X chromosome: the classical type 2 opsin genesOPN1MW and OPN1MW2. It has been suggested that women (who possess two X chromosomes) could possess multiple cone cell pigments, perhaps born as full tetrachromats who have four simultaneously functioning kinds of cone cells, each type with a specific pattern of responsiveness to different wavelengths of light in the range of the visible spectrum.[20] One study suggested that 2–3% of the world's women might have the type of fourth cone whose sensitivity peak is between the standard red and green cones, giving, theoretically, a significant increase in color differentiation.[21] Another study suggests that as many as 50% of women and 8% of men may have four photopigments and corresponding increased chromatic discrimination compared to trichromats.[20] In 2010, after 20 years of study of women with four types of cones (non-functional tetrachromats), neuroscientist Dr. Gabriele Jordan identified a woman (subject cDa29) who could detect a greater variety of colors than trichromats could, corresponding with a functional tetrachromat (or true tetrachromat).[22][23][24][25]

Variation in cone pigment genes is widespread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red/green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female new-world monkeys trichromatic vision.[26]

In humans, preliminary visual processing occurs in the neurons of the retina. It is not known how these nerves would respond to a new color channel, that is, whether they could handle it separately or just combine it in with an existing channel. Visual information leaves the eye by way of the optic nerve; it is not known whether the optic nerve has the spare capacity to handle a new color channel. A variety of final image processing takes place in the brain; it is not known how the various areas of the brain would respond if presented with a new color channel.

Mice, which normally have only two cone pigments, can be engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination,[27] arguing against some of these obstacles; however, the original publication's claims about plasticity in the optic nerve have also been disputed.[28]

Humans cannot see ultraviolet light directly because the lens of the eye blocks most light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea.[29] The photoreceptor cells of the retina are sensitive to near ultraviolet light and people lacking a lens (a condition known as aphakia) see near ultraviolet light (down to 300 nm) as whitish blue, or for some wavelengths, whitish violet, probably because all three types of cones are roughly equally sensitive to ultraviolet light, but blue cones a bit more.[30]